Don't forget to calculate your total water volume to include any external filters/sumps/refugee's you may have connected, or plan on connecting in the future. The other thing to consider is how much heat loss your aquarium can suffer... (open top? how about your filters?) When setting your temperature, calculate any heat generated by your light fixtures as well.
I have a 350w on my 90g, around 100g including external filters. My aquarium is in a basement, which I don't provide any heat to from 10pm-5pm weekdays, and 10pm-9am weekends. It keeps a stable temperature, and the heater comes on only for around 5-10 minutes per hour. It was coming on constantly previously, but since I put glass covers over most of the aquarium, and put a lid on my sump I suffer from little heat loss (and water evaporation).